Passed under and over and often overlooked, Chicago's bridges are as architecturally significant as its skyscrapers. New boutique walking tours run by civil engineer Jim Phillips explain the history, design and mechanical operation of 18 bridges spanning the Chicago River and built between 1913 and 1930. These tours aren't just for gear heads, but anyone wanting to know more about Chicago told through structural and ornamental bridge features influenced by Daniel Burnham's 1909 "Plan of Chicago." Tours start at the Beaux Arts style Michigan Avenue Bridge in the McCormick Bridgehouse and Chicago River Museum (tours can be customized). The most popular 1.5-hour tour covers 12 bridges along the lower, bank-side Riverwalk and street-level Wacker Drive and to Madison Street's Lyric Opera Bridge (return via river taxi). All these bridges -- from different architectural periods -- are modern fixed trunnion bridges, a vertical lifting bridge design dating back to Egyptian times. Jim offers extensions to the tours to include three more types of bridges in the Chinatown area and a rare bob tail swing bridge on the city's North Side. Mid-May through fall, tours are offered Friday through Monday at 1:30 p.m. Fee charged. Wear comfortable walking shoes and dress for the weather.